Thermoplastic rewelding process

ABSTRACT

Thermoplastic welding is an emerging technology targeted at significantly reducing the manufacturing cost of aerospace structure by eliminating fasteners and the touch labor associated with fasteners to prepare, install, and inspect the assemblies. Quality welds are highly dependent upon achieving appropriate temperatures everywhere along the bond line. The present invention is a system that evaluates the quality of the welds involving inputting an EM pulse to the embedded susceptor and listening to the acoustic response that the pulse generates to determine weld quality from the sound.

REFERENCE TO RELATED APPLICATION

[0001] The present application claims the benefit of U.S. ProvisionalApplication 60/025,343, filed Sep. 3, 1996.

TECHNICAL FIELD

[0002] The present invention is a nondestructive method for evaluatingthe quality and integrity of a thermoplastic weld having an embeddedsusceptor. The method uses an impulse coil to vibrate the susceptor andan acoustic sensor to listen to the vibration to assess the weldquality, generally through analysis of the return signal in thefrequency domain.

BACKGROUND ART

[0003] Composite materials lend themselves to bonded structures betterthan to fastened ones. Bonded composites have received limited use incritical aerospace structures, however, because the bonds can vary instrength or stiffness even if they have no discrete bond line defects(disbonds, porosity, voids, cracking, etc.). Traditional nondestructiveinspection methods rely upon quantifying these defects to predict theflightworthiness of the structure, but are unable to ascertain thecohesiveness of the bond at any location if defects are absent.Nondestructive identification of low strength bonds and regions of“kissing unbonds” (bonds of near zero strength) remains a significantgoal solved only in a few specific bonded applications where the resultsof shear or tensile tests have been correlated to a particular NDEsignal feature. Modified pulse-echo ultrasonic testing (UT) has beensuccessful in finding the discrete defects (voids, delaminations,porosity), but not “kissing unbonds” and low strength bonds. Infraredthermography, shearography, eddy current, and various high and lowfrequency ultrasonic methods have also been unsuccessful in discerningbond quality in thermoplastic welds.

[0004] The present invention provides a nondestructive method fortesting bond quality using an electromagnetic (EM) pulse to inducevibrations in the embedded susceptor and an acoustic receiver to listento and to record the induced vibrations. Analysis of the receivedvibration signal discriminates bond quality.

SUMMARY OF THE INVENTION

[0005] The present invention inspects bond lines that contain conductivematerial, especially those formed using a copper mesh susceptor, usinghigh energy electromagnetic pulsing with acoustic receiving in a singleinspection head to produce a unique evaluation technique. The inspectionhead contacts the outer skin of the structure whose bond is beingevaluated, and contains a pancake type copper coil containing windingswith rectangular cross sections designed to effectively couple to thesusceptor. The closing of a switch releases a charge built up in acapacitor bank, which creates a high energy pulse in the coil. Theelectromagnetic field produced by the current pulse couples to thesusceptor, opposes the driving magnetic field of the coil, and creates aforce on the susceptor. With the help of a finite element code forelectromagnetic interactions, we have been able to model the test setupand predict the forces on the susceptor. For the EM pulse produced bythe discharge of a capacitor bank (proportional to the voltage of thecoil), the normal stresses induced in the susceptor are shown in FIG. 3.The frequency of the stress peaks in the time domain is twice that ofthe voltage peaks at the coil, because a change (positive or negative)occurs on both sides of each peak.

[0006] The stresses incite vibrational modes in the susceptor andsurrounding composite, creating an acoustic wave that we receive andrecord with an electromagnetically shielded acoustic-emission (AE)transducer at the center or around the outside of the coil (FIG. 4).While the size of the signal will depend upon the size of the incomingpulse, the susceptor conductivity, and the depth of the bond line,frequency-related features of the signal can be correlated to bondquality. We have demonstrated experimentally that a susceptor that isnot fully bonded to the surrounding substructure will produce lowfrequency modes that are absent in a well bonded structure. Thedifference in the frequency response produced from a good bond and apoor bond is shown in FIGS. 5 and 6, respectively. FIGS. 5 & 6 plotFourier transforms from the time domain to the frequency domain of theultrasonic signal received at the AE transducer.

[0007] Bonds on both sides of the susceptor can be separately examined.These bonds experience both tensile and compressive stress when thesusceptor vibrates. As the vibration occurs, the bond above thesusceptor will be under tension as the bond below is under compression,and visa versa. We measure the response of the bond to a given inducedstress level, permitting a type of in-situ proof-testing of the bondquality. “Kissing unbonds” or low strength bonds can be identified withlower energy pulses.

[0008] Some radomes contain conductive layers (the FSS layers), whichmay be inspected with this device. In addition, a conductive layer canbe added to adhesive bond lines to produce an inspectable bondedstructure from an otherwise uninspectable one.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a perspective view of a moving coil thermoplasticwelding apparatus.

[0010]FIG. 2 is a schematic view, partially in section, illustrating acup coil for inducing vibrations in the susceptor of thermoplasticwelds.

[0011]FIG. 3 is a graph showing the normal stresses induced in asusceptor embedded in a thermoplastic weld by EM pulses from the coil ofFIG. 2.

[0012]FIG. 4 is a schematic graphical representation of the acousticsignal created by pulsing a susceptor in a well bonded thermoplasticweld.

[0013]FIG. 5 is a graphical representation of the acoustic signalcreated by pulsing a susceptor in an adequate strength, good qualitythermoplastic weld.

[0014]FIG. 6 is a graphical representation of the acoustic signalcreated by pulsing a susceptor in a low strength, poor qualitythermoplastic weld.

[0015]FIG. 7 is a schematic plan view of a typical susceptor tape.

[0016]FIG. 8 is a schematic representation of the maximum stress on thesusceptor as a function of position from the centerline of the coilassuming there is no offset between the coil and susceptor.

[0017] FIGS. 9A-D are graphs showing the typical correlation betweenbond strength and the area under the frequency domain Fourier Transformcurve for a low frequency response for a narrow susceptor.

[0018]FIG. 10 is a graph showing relative bond strength as a function ofthe area under the low frequency response curve.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0019] First, we will describe a typical thermoplastic weldingoperation, since these are the bonds of principle interest fornondestructive evaluation in accordance with the present invention.Then, we will describe the nondestructive evaluation system (NDE) of thepresent invention for assessing bond quality.

[0020] For purposes of this description, “laminate” means afiber-reinforced organic resin matrix composite having a plurality ofplies of prepreg or its equivalent consolidated together and cured, asappropriate. The laminates are prefabricated by any appropriate meansincluding automatic or hand tape lay up or tow fiber placement withautoclave consolidation and cure; resin transfer molding (RTM); SCRIMP;or the like. Generally, the organic matrix resin is a thermoplastic,especially PEK, PEEK, PEKK, ULTEM polyimide, or KIII. In the weldingoperation, resin in the laminates as well as resin in the susceptormelts, intermixes, and fuses to form the weld. The laminate might alsobe a thermoset in which case the welding process actually forms a hotmelt adhesive bond rather than a weld. We prefer welding, but recognizeapplication of our NDE/NDI process to assess the strength and quality ofadhesive bonds.

[0021] In a thermoplastic laminate, the reinforcing fiber typically iscarbon fiber in continuous or chopped form, and generally as tow orwoven fabric. While other fibers can be used, modern aerospacerequirements most often dictate carbon fibers for their strength anddurability, and we prefer them. In thermosets, especially epoxy, thefibers might be graphite or fiberglass.

[0022] Thermoplastic Welding

[0023] Three major joining technologies exist for joining aerospacecomposite structure: mechanical fastening; adhesive bonding; andwelding. Both mechanical fastening and adhesive bonding use costly,timeconsuming assembly steps that introduce excess cost into themanufacture of aerospace composite assemblies even if the parts arefabricated from components produced by an emerging, cost efficientprocess. Mechanical fastening requires expensive hole locating,drilling, shimming, and fastener installation, while adhesive bondingusually requires complicated surface pretreatments.

[0024] In contrast, composite welding eliminates fasteners and can jointhermoplastic composite components at high speeds with minimum touchlabor and little, if any, pretreatments. In our experience, the weldinginterlayer, called a susceptor, also can simultaneously take the placeof shims required in mechanical fastening. As such, composite weldingholds promise to be an affordable joining process. For “welding”thermoplastic and thermoset composite parts together, the resin that thesusceptor melts functions as a hot melt adhesive. If fully realized,this thermoplastic-thermoset bonding process, in addition to truethermoplastic welding, will further reduce the cost of compositeassembly.

[0025] Thermoplastic welding is a process for forming a fusion bondbetween the faying thermoplastic faces of two or more parts. A fusionbond is created when the thermoplastic on the surface of the two partsis heated to the melting or softening point and the two surfaces arebrought into contact so that the molten thermoplastic mixes. Then, thesurfaces are held in contact while the thermoplastic cools below thesoftening temperature to fuse the thermoplastic into the weld.

[0026] There is a significant stake in developing a successfulthermoplastic welding process. Its advantages versus traditionalcomposite joining methods are:

[0027] reduced parts count versus fasteners

[0028] minimal surface preparation, in most cases a simple solvent wipeto remove surface contaminants

[0029] indefinite shelf life at room temperature

[0030] short process cycle time, typically measured in minutes whenusing induction heating

[0031] enhanced joint performance, especially hot/wet and fatigue

[0032] permits rapid field repair of composites or other structures.

[0033] little or no loss of bond strength after prolonged exposure toenvironmental influences.

[0034] The exponential decay of the strength of magnetic fields withdistance from their source dictates that, in induction weldingprocesses, the structure closest to the induction coil will be thehottest, since it experiences the strongest field. Therefore, it isdifficult to obtain adequate heating at the bond line between twographite or carbon fiber reinforced resin matrix composites relying onthe susceptibility of the fibers alone as the source of heating in theassembly. For the inner plies to be hot enough to melt the resin, theouter plies closer to the induction coil and in the stronger magneticfield are too hot. The matrix resin in the entire piece of compositemelts. The overheating results in porosity in the product, delamination,and, in some cases, destruction or denaturing of the resin. To avoidoverheating of the outer plies and to insure adequate heating of theinner plies, a susceptor of significantly higher conductivity than thefibers is used to peak the heating selectively at the bond line of theplies when heating from one side. An electromagnetic induction coil onone side of the assembly heats a susceptor to melt and cure athermoplastic resin (also sometimes referred to as an adhesive) to bondthe elements of the assembly together. Often the current density in thesusceptor is higher at the edges of the susceptor than in the centerbecause of the nonlinearity of the coil. This problem typically occurswhen using a cup core induction coil like that described in U.S. Pat.No. 5,313,037 and can result in overheating the edges of the assembly orunderheating the center, either condition leading to inferior weldsbecause of non-uniform curing. It is necessary to have an open or meshpattern in the susceptor to allow the resin to bond between thecomposite elements of the assembly when the resin heats and melts.Misalignment can also result in temperature variations, producingexcessive heating in isolated locations because of the inductionphysics. U.S. patent application Ser. No. 08/565,566 describes onemechanism for achieving proper alignment between the moving inductioncoil and the susceptor to reduce problems associated with excessiveheating.

[0035] U.S. Pat. No. 4,673,450 describes a method to spot weld graphitefiber reinforced PEEK composites using a pair of electrodes Afterroughening the surfaces of the prefabricated PEEK composites in theregion of the bond, Burke placed a PEEK adhesive ply along the bondline, applied a pressure of about 50-100 psi through the electrodes, andheated the embedded graphite fibers by applying a voltage in the rangeof 20-40 volts at 30-40 amps for approximately 5-10 seconds with theelectrodes. Access to both sides of the assembly is required in thisprocess which limits its application.

[0036] Prior art disclosing thermoplastic welding with induction heatingis illustrated by U.S. Pat. Nos. 3,966,402 and 4,120,712. In thesepatents, the metallic susceptors are of a conventional type having aregular pattern of openings of traditional manufacture. Achieving auniform, controllable temperature in the bond line, which is crucial topreparing a thermoplastic weld of adequate integrity to permit use ofwelding in aerospace primary structure, but is difficult to achieve withthose conventional susceptors, as we discussed and illustrated in U.S.Pat. No. 5,500,511.

[0037] Simple as the thermoplastic welding process sounds, and as easyas it is to perform in the laboratory on small pieces, it becomesdifficult to perform reliably and repeatably in a real factory onfull-scale parts to build a large structure such as an airplane wingbox. One difficulty is in getting the proper amount of heat to the bondline without overheating the entire structure. Another is achievingintimate contact of the faying surfaces of the two parts at the bondline during heating and cooling despite the normal imperfections in theflatness of composite parts, thermal expansion of the thermoplasticduring heating to the softening or melting temperature, flow of thethermoplastic out of the bond line under pressure, and then contractionof the thermoplastic in the bond line during cooling.

[0038] a. Moving Coil Welding Processes

[0039] In U.S. Pat. No. 5,500,511, Boeing described a tailored susceptorfor approaching the desired temperature uniformity. This susceptorrelied upon carefully controlling the geometry of openings in thesusceptor (both their orientation and their spacing) to distribute theheat evenly. For example, using a regular array of anisotropic, diamondshaped openings with a ratio of the length (L) to the width (W) greaterthan 1 provided a superior weld over that achieved using a susceptorhaving a similar array, but one where the L/W ratio was one. By changingthe length to width ratio (the aspect ratio) of the diamond-shapedopenings in the susceptor, Boeing achieved a large difference in thelongitudinal and transverse conductivity in the susceptor, and, thereby,tailored the current density within the susceptor. A tailored susceptorhaving openings with a length (L) to width (W) ratio of 2:1 has alongitudinal conductivity about four times the transverse conductivity.In addition to tailoring the shape of the openings to tailor thesusceptor, Boeing altered the current density in regions near the edgesby increasing the foil density (i.e., the absolute amount of metal).Increasing the foil density along the edge of the susceptor increasedthe conductivity along the edge and reduced the current density and theedge heating. The tailored susceptor had increased foil density byfolding the susceptor to form edge strips of double thickness or bycompressing openings near the edge of an otherwise uniform susceptor.Boeing found this susceptor difficult to reproduce reliably. Also, itsuse forced careful placement and alignment to achieve the desired effectwhen using the cup coil of U.S. Pat. No. 5,313,037 and the multipasswelding process of U.S. Pat. No. 5,486,684, both of which we incorporateby reference.

[0040] With the cup coil, the magnetic field is strongest near the edgesbecause the central pole creates a null at the center. Therefore, thesusceptor is designed to counter the higher field at the edges byaccommodating the naturally higher induced current near the edges. Thehigh longitudinal conductivity encourages induced currents to flowlongitudinally.

[0041] With the tailored susceptor or with other moving coil weldingoperations, achieving the proper bond line temperature requiresempirical design calibration. Even then, the bond line temperature mayfluctuate within a relatively wide range because of misalignment,variations in the susceptor, variations in the geometry (such as skinplies or spar curvature), or variations in coil speed or coil power.Boeing has created calibration curves (i.e., allowables data) for aspecified power at a specified head speed, geometry, and materialsystem. The allowables data must be quite extensive, and there is stillno assurance that an actual run is producing a weld that corresponds tothe test data. Monitoring the bond line temperature in real time toachieve uniform temperatures at the bond line has great significance toachieving process control and quality welds. Assessing the weld qualitynondestructively is also essential since bonds of inadequate strengthwould produce catastrophe for the end product.

[0042] Boeing described a selvaged susceptor for thermoplastic weldingin U.S. Pat. No. 5,508,496. That selvaged susceptor controls the currentdensity pattern during eddy current heating by an induction coil toprovide substantially uniform heating to a composite assembly and toinsure the strength and integrity of the weld in the completed part.This susceptor is particularly desirable for welding ribs between priorwelded spars using an asymmetric induction coil of U.S. Pat. No.5,444,220, because that coil provides a controllable area of intense,uniform heating under the poles, a trailing region with essentially noheating, and a leading region with minor preheating. We incorporatethese patents by reference.

[0043] Boeing achieved better performance (i.e., more uniform heating)in rib welding by using the selvaged susceptor having a center portionwith a regular pattern of openings and solid foil edges, which it refersto as selvage edge strips. Embedding the susceptor in a thermoplasticresin makes a susceptor/resin tape that is easy to handle and to use inassembling the composite pieces prior to welding. With a selvagedsusceptor, the impedance of the central portion should be anisotropicwith a lower transverse impedance than the longitudinal impedance. Here,the L/W ratio of diamond shaped openings should be less than or equal toone. That is, unlike the tailored susceptor of U.S. Pat. No. 5,500,511,L for the selvaged susceptor of U.S. Pat. No. 5,508,496 should be lessthan W. With this selvaged susceptor, in the region immediately underthe asymmetric induction work coil described in U.S. Pat. No. 5,444,220,current flows across the susceptor to the edges where the currentdensity is lowest and the conductivity, highest.

[0044] Generally, the selvaged susceptor is somewhat wider than the bondline so that the selvage edge strips extend on either side of the bondline. Removal of the selvage edge strips after forming the weld leavesonly a perforated susceptor foil in the weld. This foil has a relativelyhigh open area fraction.

[0045] A structural susceptor allows Boeing to include fiberreinforcement within the weld resin to alleviate residual tensile strainotherwise present in an unreinforced weld. The susceptor includesalternating layers of thin film thermoplastic resin sheets and fiberreinforcement (usually woven fiberglass fiber) sandwiching theconventional metal susceptor that is embedded in the resin, and isdescribed in greater detail in U.S. patent application Ser. No.08/471,625. While the number of total plies in this structural susceptoris usually not critical, Boeing prefers to use at least two plies offiber reinforcement on each side of the susceptor.

[0046] The structural susceptor permits gap filling between the weldedcomposite laminates which tailors the thickness (number of plies) in thestructural susceptor to fill the gaps, thereby eliminating costlyprofilometry of the faying surfaces and the inherent associated problemof resin depletion at the faying surfaces caused by machining thesurfaces to have complementary contours. Standard manufacturingtolerances produce gaps as large as 0.120 inch, which is too wide tocreate a quality weld using the conventional susceptors.

[0047] Boeing can easily tailor the thickness of the structuralsusceptor to match the measured gap by scoring through the appropriatenumber of plies of resin and fiber reinforcement and peeling them off.In doing so, a resin rich layer will be on both faying surfaces and thislayer should insure better performance from the weld.

[0048] To form a structural susceptor, Boeing takes a barbed susceptorand loosely bonds fiberglass reinforcing fiber and thermoplastic filmsin alternating layers symmetrically on both sides, similar to what isshown in U.S. patent application Ser. No. 08/471,625. The fiberglassreinforcement prevents the resin from fracture under the residual strainleft after welding. Higher ductility resins such as PEEK, PEK, and ULTEMpolyimide also resist fracture better than some thermoplastics. Thethermoplastic films are preferably the same resin as that used to embedthe metal foil and to fabricate the laminates. Sheet thicknesses forthese films are usually about 0.001-0.002 inch (0.025-0.050 mm). Thewoven fibers are preferably oriented perpendicular and parallel to thelongitudinal axis of the weld.

[0049] The structural susceptor is generally loosely bonded together byheat or pressure or both, but could be of essentially unitaryconstruction if desired. Being loosely bonded helps in gap filling.Boeing uses at least two layers of fiber and thermoplastic on each sideof the susceptor, but the absolute number is not critical. Boeing testedfour different styles of fiberglass and achieved similar results witheach, so the type or style of fiberglass does not seem to be critical.

[0050] The fiber suppresses cracking if the fiber volume is at leastabout 30%. The thermoplastic ensures a resin rich weld.

[0051] Described in greater detail in U.S. patent application Ser. No.08/469,604, which we incorporate by reference, “smart” susceptors aremagnetic alloys that have high magnetic permeability's but that alsohave their magnetic perineabilities fall to unity at their Curietemperature. At the Curie temperature, then, the susceptors becomeinefficient heaters. The alloys are selected to have Curie points closeto the process temperature of welding and have low thermal expansioncoefficients to match composites. The preferable alloys for thisapplication are in a composition range of from 36% Ni to 44% Ni in Fe.Additional alloying elements such as Al, Cb and Ti allow these lowexpansion iron-nickel alloys to be age hardened and add to the cap/skinpulloff strength.

[0052] The need for a susceptor in the bond line poses many obstacles tothe preparation of quality parts. The metal which is used because of itshigh susceptibility differs markedly in physical properties from theresin or fiber reinforcement, so dealing with it becomes a significantissue. A reinforced susceptor, which is described in U.S. patentapplication Ser. No. 08/469,986, overcomes problems with conventionalsusceptors by including delicate metal foils (0.10-0.20 inchwide×0.005-0.010 inch thick; preferably 0.10×0.007 inch) in tandem withthe warp fibers of the woven reinforcement fabric. The woven arrangementholds the foils in place longitudinally in the fabric in electricalisolation from each other, yet substantially covering the entire widthof the weld surface. This arrangement still allows adequate space forthe flow and fusion of the thermoplastic resin. Furthermore, in the bondline, the resin can contact, wet, and bond with the reinforcing fiberrather than being presented with the resin-philic metal of theconventional systems. There will be a resin-fiber interface with onlyshort runs of a resin-metal interface. The short runs are the length ofthe diameter of two weave fibers plus the spatial gap between the weavefibers, which is quite small. Thus, the metal is shielded within thefabric and a better bond results. In this woven arrangement the foil canassume readily the contour of the reinforcement. Finally, thearrangement permits efficient heat transfer from the foil to the resinin the spatial region where the bond will form.

[0053] Conventional susceptors are essentially planar (X-Y) metal sheetsor laminates of planar films. Welds that embed these susceptors lackreinforcement in the Z-plane, but welds can include such reinforcement(with corresponding improvement in the pulloff strength) if theyincorporate a barbed susceptor of U.S. patent application Ser. No.08/486,560. A barbed susceptor typically uses a Fe—Ni alloy susceptorthat is formed to include barbed, Z-pin reinforcement to provideimproved pulloff strength. The alloy chosen for this susceptor has acoefficient of thermal expansion(CTE) that essentially matches the CTEof the composite and a Curie temperature of about 700° F. (370° C.),which is essentially ideal for thermoplastic welding of resins likeKIIIA polyimide since it is slightly above the resin's melt temperature.For this application, an alloy of 42% Ni-58% Fe including γ′strengthening elements of Al, Ti and Cb yields both low CTE and highstrength. The susceptor is preferably made by laser cutting a foil ofthe material to form barbed tabs and pushing the cut tabs alternately upand down to give the susceptor a three dimensional character.Alternatively a woven wire mesh may be used in this application withalternating wires extending in the Z direction. The thermoplastic resincures or consolidates around the barbs during the welding process whichprovides the pulloff strength improvement.

[0054] The barbed susceptor of U.S. patent application Ser. No.08/486,560 usually is fabricated from an age-hardened Invar foil havinga thickness of from 0.003-0.010 inch (0.075-0.25 mm). It may be madefrom other materials having good electrical conductivity and highmagnetic permeability. The susceptor may have a pattern of openings madeby forming barbs in the Z-axis by folding prongs out of the X-Y plane.The result is a susceptor that resembles barbed wire. Each prong of thesusceptor might also be barbed like a fishhook. Such barbs are readilyformed simply by scoring the prong with a cut that starts relativelycloser to the body of the susceptor and extends into the prong at anangle running from the surface toward the tip. This Invar susceptor is“smart”, and helps to avoid excessive heating, because of its Curiepoint.

[0055] The barbed susceptor may also have a pattern of openings in theX-Y plane with uniform line widths of about 7 mils (0.18 mm) to definethe peripheries of the diamond, as the other susceptors do, so that afusion bond can occur through the susceptor. Of course, the openings canhave shapes other than diamonds. The diamonds are easy to form byetching, stamping, or expanding and provide a convenient mechanism tocontrol the longitudinal and transverse impedance, as described inBoeing's other patent applications. The diamonds can have L/W ratiosless than or equal to 1.0 in the selvaged susceptor where Boeing wasinterested in influencing the eddy currents to run transversely into thesolid edge strips. Other shapes can be used for the openings to create afoil that has a uniform impedance or whatever desired ratio in thelongitudinal and transverse directions.

[0056] The barbed susceptor might be a “reinforced” multistrip susceptorsimilar to that described in U.S. patent application Ser. No. 08/469,986with the strips being periodically cut to create Z-plane barbs. Thismultistrip concept may actually be best suited for resistance weldinglike that described in U.S. patent application Ser. No. 08/470,168 orheating in our induction solenoid coil heating workcell of U.S. Pat.Nos. 5,624,594 or 5,641,422, because these two processes induce currentsthat run longitudinally through the susceptor. The multistrip susceptorhas low longitudinal impedance.

[0057] Welding researchers have devoted significant effort to developinductor and susceptor systems to optimize the heating of the bond linein the welded thermoplastic assemblies. Another hurdle remaining toperfect the welding process to the point of practical utility forproducing large scale aerospace-quality structures in a productionenvironment is the aspect of the process dealing with the control of thesurface contact of the faying surfaces. This aspect of thermoplasticwelding controls the timing, intensity, and schedule of heatapplication. The material at the faying surfaces is brought to andmaintained within the proper temperature range for the requisite amountof time for an adequate bond to form. Then, intimate contact ismaintained while the melted or softened material hardens in its bondedcondition.

[0058] Large scale parts, such as wing spars and ribs, and the wingskins that are bonded to the spars and ribs, are typically on the orderof 20-30 feet long at present, and potentially, can be several hundredfeet in length when the thermoplastic welding process is perfected forcommercial transport aircraft. Parts of this magnitude are difficult toproduce with perfect flatness. Instead, the typical part will havevarious combinations of surface deviations from perfect flatness,including large scale waviness in the direction of the major lengthdimension, twist about the longitudinal axis, dishing or sagging of “I”beam flanges, and small scale surface defects such as asperities anddepressions. These irregularities interfere with full surface areacontact between the faying surfaces of the two parts and can result insurface contact only at a few “high points” across the intended bondline. Additional surface contact can be achieved by applying pressure tothe parts to force the faying surfaces into contact, but full intimatecontact is difficult or impossible to achieve in this way. Applying heatto the interface by electrically heating the susceptor in connectionwith pressure on the parts flattens the irregularities when the resinmelts. Additional time is needed after flattening to achieve fullintimate contact. Extended use of heat and pressure may be excessive,however, and may result in deformation of the top part. When the overalltemperature of the “I” beam flange is raised to the softening point, itwill begin to yield or sag under the application of the pressure neededto achieve a good bond. If sagging occurs the necessary pressure will belost and so will the final product configuration.

[0059] Boeing's multipass thermoplastic welding process described inU.S. Pat. No. 5,486,684 enables a moving coil welding process to producecontinuous or nearly continuous fusion bonds over the full area of thebond line to yield high strength welds reliably, repeatably, and withconsistent quality. This process produces improved low cost, highstrength composite assemblies of large scale parts, fusion bondedtogether with consistent quality. It applies heat according to aschedule that melts the resin at the faying surfaces yet maintains theoverall temperature of the structure within the limit in which itretains its high strength. It avoids sagging and, so, does not requireinternal tooling to support the structure against sagging whichotherwise could occur above the high strength temperature limit. Theprocess also produces nearly complete bond line area fusion on standardproduction composite material parts having the usual surfaceimperfections and deviations from perfect flatness. The welding processeliminates fasteners and the expense of drilling holes, inspecting theholes and the fasteners, inspecting the fasteners after installation,sealing between the parts and around the fastener and the holes;reducing mismatch of materials; and arcing from the fasteners.

[0060] In the process, an induction coil is passed multiple times over abond line while applying pressure at least in the region of the coil tothe assembled components to be welded and maintaining the pressure untilthe resin hardens. The resin at the bond line is heated to the softeningor melting temperature with each pass of the induction coil and pressureis exerted to flow the softened/melted resin in the bond line and toreduce the thickness of the bond line while improving the intimacy ofthe faying surface contact with each pass. Multiple passes then completethe continuity of the bond. The total time at the softened or meltedcondition of the thermoplastic in the faying surfaces is sufficient toattain deep inter diffusion of the polymer chains in the materials ofthe two faying surfaces throughout the entire length and area of thebond line. Doing so, produces a bond line of improved strength andintegrity in the completed part. Because the total time of the fayingsurfaces at its softening temperature is separated into severalsegments, heat in the interface dissipates between passes so that eachsubsequent pass reheats the resin at the faying surfaces but does notraise the temperature of the entire structure to the degree at which itloses its strength and begins to sag. The desired shape and size of thefinal assembly is maintained.

[0061] Another moving coil welding operation seeks to apply asubstantially constant and uniform pressure on the entire bond linethroughout the welding operation. As described in U.S. patentapplication Ser. No. 08/367,557, such a welding operation, which Boeingcalls “fluid tooling,” includes an elongated vessel made of fluidimpervious flexible material. The vessel has an elongated axis and anopen end at each axial end of the vessel, and has a cross sectionaldimension sized to accommodate the coil. Each axial end of the vessel isclosed and sealed by an end closure. At least one of the end closures isremovable for insertion of the coil into the vessel. A linear guide inthe vessel extends axially for substantially the full length of thevessel and guides the coil for movement axially through the vessel.Power leads are connected to the coil and extend through a pass-throughin one end closure to connect the coil to a source of high frequencyelectrical power to energize the coil to produce an alternating magneticfield. A motive system is provided for moving the coil axially along thevessel over the bond line at a controlled speed. The motive systemgenerally includes a pair of magnets guided along opposite sides of thevessel and magnetically coupled to a ferromagnetic mass connected to thecoil. The magnets are moved along their guides and pull the coilattached to the ferromagnetic mass inside the vessel. A backup structureexerts a downward force along the top of the vessel, pressurizing fluidsealed in the vessel and distributing the pressure uniformly over thetop surface of the top part to press the top part against the bottompart and facilitate fusion bonding of the thermoplastic in the fayingsurfaces of the interface.

[0062] b. Fixed Coil Induction Welding

[0063] Boeing has also experimented with thermoplastic welding using itsinduction heating workcell, and, of course, discovered that the processdiffers from the moving coil processes because of the coil design andresulting magnetic field. The fixed coil workcell presents promise forwelding at faster cycle times than the moving coil processes because itcan heat multiple susceptors simultaneously. The fixed coil can reduceoperations to minutes where the moving coil takes hours. The keys to theprocess, however, are achieving controllable temperatures at the bondline in a reliable and reproducible process that assures quality weldsof high bond strength. Boeing's fixed coil induces currents to flow inthe susceptor differently from the moving coils and covers a largerarea. Nevertheless, Boeing has developed processing parameters thatpermit welding with its induction heating workcell using a susceptor atthe bond line. The fixed coil process is described in greater detail inU.S. Pat. No. 5,624,594, which we incorporate by reference.

[0064] Another advantage with the fixed coil process is that welding canoccur using the same tooling and processing equipment used toconsolidate the skin, thereby greatly reducing tooling costs. Finally,the fixed coil heats the entire bond line at one time to eliminate theneed for shims that are currently used with the moving coil. Boeing cancontrol the temperature and protect against overheating by using its“smart” susceptors as a retort or as the bond line susceptor material orboth.

[0065] C. Temperature Monitoring

[0066] In U.S. patent application Ser. No. 08/548,823 Boeing describes asystem for thermoplastic welding to monitor the bond line temperature inreal time allowing detection of the onset of flow of the thermoplasticresin. The system permits guidance control of the induction head toadjust its power, speed, or motion in response to the measuredtemperature. Basically, Boeing embeds at least one multinodethermocouple within the weld near the bond line in a layer adjacent thesusceptor to measure the temperature under the moving coil.

[0067] The thermocouple is made by twisting the wires together or in azig-zag fashion to form periodic nodes along the bond line. A singlewire thermocouple configuration using constantan wire and using thecopper susceptor as the second conductor also possible. The spacing ofthe nodes depend on the desired resolution, but, should be about 0.2inch or so apart.

[0068] The thermocouple will be an open circuit prior to the onset ofthermoplastic flow, and will not have a voltage output. At the onset offlow, the two thermocouple wires short and produce a thermoelectricvoltage proportional to the temperature of the thermocouple junction.The thermocouple will read the temperature directly under the inductionhead, that being the hottest junction and also the one that is closestto the monitor input. The multinode thermocouple behaves like a seriesof parallel batteries. The node closest to the monitor produce thehighest voltage amplitude because it directly in the hot zone. The samenode also acts as a short to any other voltages produced by thermocouplenodes further away from the monitor. Each consecutive junction shortsthe potential generated by the preceding node. If the node contactresistance is high there may be a small error.

[0069] As described in U.S. patent application Ser. No. 08/548,823Boeing welded a test panel with a sliding junction (multinode)thermocouple in the bond line. The thermocouple was made with two bareChromel/aluminel, AWG #36 wires and wound in a zig-zag way on a piece ofthermoplastic resin or was encapsulated with the resin. The thermocouplewas located half way between the center of the bond line and the edge.Boeing also welded a second test panel with two multinode thermocouplesnear edges of the susceptor in the bond line. The thermocouples werelocated half way between the center and the edge on each side of thebond line, with nodes spaced one inch apart. The output of the twothermocouples tracked within 25° F.

[0070] By locating the thermocouples on the outer edges of the bondline, the voltages generated by the two thermocouples produce a guidancecontrol function formed by combining the two thermocouple outputs with adifferential amplifier bridge circuit. When the coil moves off center,it will produce uneven heating across the bond line. This heating willresult in a differential thermocouple output signal used to restore thecoil to the center of the susceptor, and, thereby, restore uniformheating across the bond line. Nevertheless, there also remains problemswith the accuracy of positioning in the assembly, with shorting, andwith reproducibility in what currently is a task requiring relativelyhigh skill.

[0071] A drawback to this multinode thermocouple method of processmonitoring and control for induction welding is that it is intrusive.The thermocouple wires remain in the bond line. The diameter of thethermocouple wires are as small as 0.001 inch. They should not presentsignificant structural problems. The insulation of the thermocouple wireshould be the same thermoplastic resin as that being welded and shouldnot have any adverse effect on the structural properties of the bond.

[0072] In U.S. Pat. No. 5,573,613, Boeing also described a method fordetermining the susceptor temperature by measuring the change inimpedance of the induction coil. As the susceptor heats, its electricalresistance changes as a function of the thermal coefficient ofresistance (TCR) of the susceptor material, and that change is reflectedback as a change in the drive coil impedance. An electrical circuitsenses the varying impedance/resistance and converts that change into achange of temperature on a temperature display, or into a signal toadjust the power to the coil or the speed of travel of the coil alongthe bond line. The sensing circuit includes a high power bridge with asensitive null arm to sense changes in the susceptor impedance due totemperature changes.

[0073] A simple L-R bridge detects the changing resistance of thesusceptor as its temperature changes during inductive heating. Thebridge includes a high-power transformer of about 500 watts operating atabout 35-55 kHz connected across a pair of series-connected inductors L₁and L₂ and a pair of series-connected resistors R₁ and R₂. Bothseries-connected pairs are connected to each other in parallel and inparallel with the transformer. A shunt with a voltage sensor (such as avoltmeter or an oscilloscope) is connected between the two resistors andthe two inductors. The two sides of the bridge are asymmetric by atleast 2:1 to put most of the power in the bond line for the sake ofefficiency, since power dissipated in the reference side of the bridgeis wasted. The two coils L₁ and L₂ are designed to track fairly closelyso that their inductances and Q's (i.e. the dimensionless power ratio ofstored to dissipated power) vary consistently with frequency. One ofinductors L₁ or L₂ is the moving coil to transfer energy to thesusceptor.

[0074] The bridge signal is used to control the welding processinteractively by adjusting the power to the coil in a closed loop RFheating control circuit, or by adjusting the speed of travel of the coilover the bond line, or both, so as to maintain the melt pool temperaturewithin the desired range of optimum processing temperature, that is,

[0075] 620±25° F. in the case of the DuPont Avamid KIIIB polyimide. Thesignal is conditioned in a suitable conditioning circuit, which woulddepend on the voltage sensor used and could produce a digital signal tothe power amplifier to turn the amplifier up or down, in the nature of athermostat control, whenever the melt pool temperature drops below orexceeds the optimal temperature range. Preferably, the signalconditioner circuit produces a signal proportional to the voltage sensorsignal to adjust the power to the work coil up or down from apredetermined average power level known to maintain a steady statetemperature in the melt pool at the coil speed used. Nevertheless,overheating can still be a significant problem, especially if localizedoverheating arises from misalignment between the moving coil and thesusceptor.

[0076] d. Steering the Moving Coil

[0077] A nonintrusive system associated with a moving induction coil,particularly one of the type described in U.S. Pat. No. 5,313,037,self-steers the coil over the susceptor to avoid excessive, damagingoverheating that otherwise might occur because of misalignment betweenthe coil and the susceptor. Alternately, the system can sense themisalignment by the aberration in the magnetic field and can create acompensating “hot spot” with a differential, parasitic, secondary coil.

[0078] In U.S. patent application Ser. No. 08/565,566, when there is amisalignment between the primary coil of the induction head and thesusceptor, the self-steering system produces a guiding command with asecondary coil to return the primary coil to the centerline.Alternately, the system can use a differential, parasitic secondary coilto compensate for the misalignment and to achieve better temperatureuniformity in the bond line by adjusting the magnetic field. Toaccomplish these features, Boeing uses two, peripheral coils connectedin differential mode to produce a null (i.e., no differential voltage)when the coil is centered over the susceptor. Doing so, Boeing tips thecoils at 45° on the sides of the cup coil of U.S. Pat. No. 5,313,037.

[0079] A compensating secondary coil can be located in the centerline ofthe drive coil. This secondary coil has a “lazy 8” design and producesno measurable effect when inserted between the primary coil and theparts assembled for welding, unless the coil and susceptor aremisaligned. When there is misalignment, the “lazy 8” forms acompensatory “hot spot” on the side of the susceptor that wouldotherwise be cool because of the misalignment. Compensation occursprovided that the “lazy 8” has a total resistance lower than the eddy,but the effect does not fully compensate for the offset.

[0080] Evaluating the Quality and Integrity of the Thermoplastic Weld

[0081] Turning now to FIG. 1, a thermoplastic welding head 10 thatincludes leading and trailing pneumatic pressure pads and a primaryinduction coil 25 disposed between the pads is supported on toolingheaders 12 over thermoplastic composite parts to be fusion bondedtogether. The parts, in this example, include a thermoplastic spar 14and a thermoplastic wing skin 16, only a small section of which is shownin FIG. 1. The spar 14 is in the form of an “I” beam having a top cap18, a bottom cap 20, and a connecting web 22. The spar 14 extendslengthwise of the wing of the airplane for which the parts are beingassembled, and the wing skin is bonded over the full length and surfacearea of the spar cap 18 with sufficient strength to resist the tensileand peeling forces the wing will experience in flight. The apparatusshown is more fully described in U.S. Pat. No. 5,556,565. The beamsmight be all composite construction or a hybrid metal webbed compositecapped beam as described in U.S. Pat. No. 5,556,565. We could also jointhermoset skins and spars with a hot melt thermoplastic adhesive.

[0082] A copper mesh susceptor 32 (i.e., a metal foil 702 susceptible toinduction heating encapsulated in a thermoplastic resin 704, FIG. 7) isinserted between the spar cap 18 and the wing skin 16. Typically theencapsulating resin is the same or a slightly lower melting temperatureformulation of the same thermoplastic resin of the spar cap 18 and thelower faying surface of the wing skin 16.

[0083] The welding head 10 can be any moving coil apparatus that iscapable of applying pressure during induction heating of the bond lineto promote fusion and after heating for a period sufficient for theresin to cool and harden in its bonded condition. Suitable welding headsare disclosed in U.S. Pat. Nos. 5,635,094; 5,444,220; and 5,313,037. Apreferred welding apparatus includes an induction coil 25 for inducingeddy currents in the susceptor 32. The eddy currents heat the susceptorby electrical resistance heating and soften or melt the thermoplasticresin in the faying surfaces of the parts so it flows, interdiffuses,and fuses together with softened resin of the wing skin and spar capupon cooling.

[0084] The coil shown in the '037 patent provides zero eddy current atthe center with the current density increasing toward the edges. Use ofa tailored susceptor is desirable to counterbalance the nonuniform eddycurrent density that the coil produces from centerline to edge toachieve uniform heating, and such a susceptor is disclosed in U.S. Pat.No. 5,500,511. A selvaged susceptor designed especially for use with theasymmetric induction coil of U.S. Pat. No. 5,444,220 is described inU.S. Pat. No. 5,508,496.

[0085] The primary induction coil 25 is mounted in the welding head 10in the center of a lower frame which is pinned to a link connecting thelower frame to an upper frame. The upper frame is pulled by a motiveapparatus including a stepper motor driving a drive sprocket and a chainloop through a reduction gear unit. A pair of camroll bearings projectsfrom both sides of the lower frame into cam grooves milled into theinside surfaces of the headers to guide and support the lower frame. Asimilar set of camroll bearings projects outward from the upper frameinto a straight cam groove to guide the upper frame as it is pulled bythe chain loop from one end of the wing skin to the other.

[0086] The process of welding the wing skin to the spar cap begins withassembling the parts together with the susceptor 32 interposed betweenthe faying surfaces of the parts. In the case of a wing box, we attachthe susceptor 32 to the outer surfaces of the spar caps 18 and 20 andthen sandwich the spars between the upper and lower wing skins 16. Theparts are held in position and squeezed together by a force exerted by apair of air bearing pads to which air under pressure is delivered by wayof air lines and distributed to the air bearing pressure pads byseparate air lines. The air to the pads reduces the frictional drag onthe pressure pads on the top surface of the wing skin and helps to coolthe parts after the coil has passed. The induction coil 25 moves alongthe intended bond line over the outer surface of the wing skin ingeneral alignment (±0.125 in) with the susceptors while producing analternating magnetic field which projects through the wing skins andaround the susceptor, generating eddy currents in the susceptor. Theeddy currents induced by the magnetic field are of sufficient amperageto heat the susceptor, raising the temperature of the thermoplasticmaterial in the faying surfaces to its softening or melting temperature.After the first pass of the welding head over each bond line to seal thebox, the process is repeated three or more times, usually increasing thepower to the coil after the second pass and, if desired, increasing thepressure exerted by air cylinders on the pressure pads.

[0087] The bond strength improves with multiple passes of the weldinghead over the same bond line. Multiple passes of the induction coilserves to create the optimal conditions for achieving a fusion bond withthe desired characteristics of continuity over the entire bond line, andsubstantial molecular interdiffusion of the materials in the fayingsurfaces to produce a bond line of high pulloff strength with thecomplete or nearly complete absence of voids, as discussed in U.S. Pat.No. 5,486,684. Welds having higher pulloff strengths use a barbedsusceptor of U.S. patent application Ser. No. 08/486,560 on the bondline.

[0088] The mechanisms for achieving a fusion bond include intimatecontact and “healing.” Intimate contact of the two faying surfaces is afunction of force exerted on the parts to squeeze them together, andtemperature-dependent viscosity. The force exerted on the parts isdistributed over a certain surface area as interfacial pressure tendingto bring the faying surfaces together. The viscosity of the surfacematerial is manifested by the tendency of high spots in the surface toyield of flow so that low spots in the two surfaces can come together.“Healing” is partly a process in which molten or softened materials flowtogether and blend where they come into contact, and partly a process ofmolecular penetration of the polymer chains in the material of onesurface into the molecular matrix of the material in the other fayingsurface. The average penetration distance of the polymer chains, withoutthe beneficial mixing effect achieved by flowing the materials in thefaying surfaces, increases as a quarter power of time (i.e., t^(0.25)).

[0089] Objective and easily made observations of a bond line that areindicative of “healing” of the quality of the bond are reduction in bondline thickness, improved ratio of bonded to unbonded surface area in thebond line (or expressed conversely, a reduction of the amount ofunbonded surface area in the bond line), and improved pass-through of abonding resin through openings in the susceptor.

[0090] Irregularities, such as hollows, depressions, and asperities(i.e., peaks) in the faying surfaces of the parts, and other deviationsfrom perfect flatness can interfere with and prevent continuous intimatecontact along the full surfaces of the parts where bonding is intended.These deviations from perfect flatness include small scale surfacefeatures such as asperities, depressions or hollows, scratches andbumps, and also large scale features such as waviness in the directionof the major length dimension, twist about the longitudinal axis,dishing or sagging of “I” beam flanges, and warping such as humping orbowing in the longitudinal direction. The structural susceptor isparticularly suited for dealing with these problems.

[0091] Boeing's goal is to produce aircraft structure that eliminatesfasteners. Welded structure will be far less expensive because weldingeliminates the labor to drill holes accurately and to inspect thefasteners after installation. We also will avoid other problems thatfasteners introduce, such as sealing around the fastener and the holes,mismatch of materials, and arcing from the fasteners. To replace thefasteners, however, requires confidence that the welds are uniform andconsistent. A failure at any weak point in the weld could lead tocatastrophic unzipping of the entire welded structure. One of the mostimportant problems with quality welding is temperature uniformity alongthe bond line to achieve uniform and complete melt and cure of theresin. Being a “smart” susceptor, our barbed susceptor has a Curietemperature slightly higher than the welding temperature (i.e., about700° F.) so the possibility of disastrous overheating is reduced. Thepresent invention is a reliable method to test the weld quality able todistinguish low strength, inadequate bonds from well bonded welds.

[0092] Boeing embeds the foil in the resin to simplify the weldingprocess. Making a foil/resin tape eliminates the steps of applyingseparate layers of resin between the respective elements in acomposite-susceptor-composite assembly. It also ensures that there willalways be adequate resin proximate the susceptor and essentially uniformresin thickness across the welding bond line. The typical tape is about2-4 inches wide with KIIIA Avimid resin (an aromatic polyimide),although the resin can be PEEK, PEKK, PES, PEK, ULTEM, or any otherthermoplastic. The resin must be compatible with the matrix resin in thecomposite and generally is the same resin as the matrix resin whenwelding thermoplastic composites. For the “welding” analog for thermosetcomposites, the resin will likely be a comparable thermoplasticformulation of the matrix resin in the composites or a compatible resin.

[0093] As shown in FIG. 2, the transmitter-receiver 200 of the presentinvention is similar in overall design to the cup coil of U.S. Pat. No.5,313,037. The transmitter-receiver, nonetheless, is adapted for thenondestructive evaluation (NDE) method of the present invention toproduce an electromagnetic pulse to vibrate the susceptor and to receivethe returning acoustic signal representing the susceptor vibration. Thetransmitter-receiver 200 has a housing 205 which contains the cup(pancake) coil 210 similar to that described in U.S. Pat. No. 5,313,037around a central pole 215. Unlike the patented cup coil, however, thecentral pole 215 in the present invention carries a shielded acousticemission transducer (receiver) 220 at the center of the pole. Thetransmitter-receiver 200 may also include active cooling plumbing forcirculating cooling water or other suitable coolant around the pancakecoil 210 during its operation, analogous to the cooling circuit forBoeing's induction coil. Applicants generally do not include thiscooling plumbing.

[0094]FIG. 2 also illustrates that the pancake coil 210 is connected toa 240 μF capacitor bank 225 and high voltage D.C. power supply 230 sothat an electromagnetic pulse of predetermined characteristics (i.e.,time, energy, frequency, and amplitude) can be introduced to the coil210 by activating the switch 235. The power supply typically suppliespower in the range from 0-10 kV, and we prefer 500 V. The pulse to thecoil has the characteristics generally shown in FIG. 4 with a durationof about 0-10 msec at 500±0.1-5.0 V.

[0095] The receiver circuitry is also shown in FIG. 2, and includes adigitizing oscilloscope 240 connected to the acoustic emission receiver220 for viewing the acoustic signal representing the susceptorvibration; a computer processor 245 for transforming the acoustic signalfrom the time domain to the frequency domain or to provide othersuitable signal processing to allow discrimination of the weld quality;and a printer 250 for plotting the various evaluation results. Thesensitivity of the receiver is a function of the pulse current, thedistance through the composite to the susceptor, the thickness of thecomposite, and the conductivity of the susceptor and composite. Thecurrent and distance are factors because they represent the strength ofthe induced magnetic field reaching the susceptor. The compositethickness is a factor because the returning acoustic signal must travelthrough the composite. The conductivities are also related to themagnetic field strength at the susceptor. We have not discovered anydifferences in performance because of the use of different resins in thecomposite or the bond.

[0096] As shown in FIG. 2, the transmitter-receiver 200 is positionedover the welded assembly 300 (such as a composite skin to composite sparweld) to pulse the susceptor 32 and to receive the resulting acousticsignal that the vibrating susceptor creates. The acoustic emissionreceiver 220 is generally located above the centerline of the susceptor32 inside (substantially concentric with) or outside the windings of apancake coil 210. The AE receiver might also be located on the oppositeface of the assembly if access to both sides of the assembly ispossible. In most of our tests we used this alternate “through assembly”arrangement, but we prefer the transmitter-receiver arrangement shown inFIG. 2 for assessing authentic aerospace structure where blindsideaccess generally is impractical or unavailable. An EM pulse from thepancake coil 210 penetrates through the composite skin 255 and into theweld 260 where it induces eddy currents in the susceptor 32. The pulsemay involve the lower composite spar or web cap 265 without significantinteraction with the composite.

[0097] To conduct a test, we step the transmitter-receiver incrementallyalong the bond line limited only by the time required to recharge thecapacitor bank, which is quite fast. For an automated inspection, wewould couple the transmitter-receiver to a stepper motor or othersuitable motive means to convey the transmitter-receiver incrementallyover the bond line. At each pulse, the transmitter-receiver isstationary.

[0098] We believe that our test method should work with continuous orsegmented susceptors and with dispersed microparticles at the bond line,although our tests have focused on narrow (2-3 inch) and wide (4-5 inch)copper susceptors having an expanded diamond pattern or etched squarepattern of openings, like those described in Boeing's patents. We havenot tested a bond line reinforced with Z-pins, but we expect our methodto work there as well. The Z-pins may alter the characteristic returnsignal, however, by modulating the susceptor's vibrational modes.

[0099] When pulsed, the susceptor 32 experiences stresses that translateinto an acoustic signal 400 representative of the susceptor vibrationthat results because of the stresses. FIG. 3 shows the theoreticalstress on the susceptor created by a 20 mV, 15.6 kamp pulse of about0.0001 sec duration. This stress produces an acoustic signal from thecompression and tension of the weld resin that propagates through theassembly's composites to the AE receiver 220. FIG. 4 shows a typicalanalog acoustic signal in the time domain (i.e., amplitude v. time),which we transform the acoustic return signal using a suitable Fouriertransform algorithm or other suitable transform to the frequency domain(i.e., amplitude v. frequency). Our signal processing is applicable toanalog or digital representations of the acoustic vibration, althoughanalog processing probably is simpler. In the frequency domain 500, weare able to discriminate between welds of adequate strength and those ofdangerously low or no strength. In particular, FIG. 5 shows the typicalspectral response for a weld having adequate strength while FIG. 6 showsthe spectral response for a weld having inadequate strength. Thesignificant characteristic in the spectral response between an adequateweld and an inadequate, low strength weld is the presence of lowfrequency peaks 605 in the spectral response in the 1-3 kHz range forthe inadequate welds. The precise location of this low frequency signaldepends upon the geometry of the part under test and the susceptor, but,for all configurations, we have been able to distinguish quality weldsfrom low strength welds or bonds by finding this low frequency peak inthe return of the low strength bonds.

[0100]FIG. 8 shows the relationship of stress in the susceptor as afunction of position displaced from the centerline when a pulse istransmitted with our pancake coil transmitter and the return signal isreceived at the center of the coil. The stress is bimodal and issymmetrical about the central pole of the coil and centerline of thesusceptor. That is, δ=0.

[0101] Alignment between the transmitter-receiver and susceptor does notappear to be a critical concern, which makes our method easier to use.We achieve the best results, however, when the receiver is about 1 inchlaterally from the transmitter. The vibrating susceptor creates adispersive, global acoustic signal in the composite. The pulse, being soshort in duration, apparently does not heat the susceptor or bond linesignificantly.

[0102] The area under the vibration signal curve for frequencies up to 1kHz provides a convenient correlation for the strength of the bond. Thebond strength is inversely proportional to the area. FIGS. 9A-D showbonds of four different strengths, the frequency response curve up to 1kHz, and the area under the frequency response curve. The correlationfor a narrow susceptor I-beam weld. Values of strength are normalizedrelative to a reference strength and the normalized plot of FIG. 10shows a substantially linear degradation of relative bond strength asthe area under the response curve increases.

[0103] Narrow susceptors exhibit essentially Mode 1 vibrations. Widersusceptors may vibrate differently, so the bond strength v. areacorrelation may not hold for wider susceptors.

[0104] While we have described preferred embodiments, those skilled inthe art will readily recognize alterations, variations, andmodifications which might be made without departing from the inventiveconcept. Therefore, interpret the claims liberally with the support ofthe full range of equivalents known to those of ordinary skill basedupon this description. The examples are given to illustrate theinvention and not intended to limit it. Accordingly, limit the claimsonly as necessary in view of the pertinent prior art.

We claim:
 1. A nondestructive method for evaluating the integrity andstrength of a thermoplastic weld or adhesive bond having an embeddedsusceptor, comprising the steps of: (a) transmitting an electromagneticpulse from an induction coil to the susceptor to create ultrasonicvibrations in the susceptor; (b) receiving an acoustic signalrepresenting the ultrasonic vibration of the susceptor; and (c)analyzing the received vibration signal to assess the thermoplastic weldquality.
 2. The method of claim 1 wherein the step of analyzing includestransforming the signal to the frequency domain and discriminating lowstrength bonds by the presence of low frequency vibrations.
 3. Anondestructive method for evaluating the quality of a bond between twosurfaces, comprising the steps of: (a) inducing vibrations in asusceptor embedded within the bond; (b) receiving an acoustic signalrepresenting the induced vibrations; and (c) analyzing the signal todeduce the quality of the bond.
 4. A transmitter-receiver for thenondestructive evaluation of the integrity and strength of athermoplastic or an adhesive bond, comprising: (a) a ferromagnetic cupcore having a central pole and an annular collar; (b) a pancake coilwound around the central pole; and (c) an acoustic-emission transducerinside or outside the coil.
 5. The transmitter-receiver of claim 4further comprising: a pulse generator electrically connected to the coilfor generating high energy electromagnetic pulses in the coil atpredetermined intervals.
 6. A system for evaluating the integrity andstrength of a thermoplastic weld or adhesive bond having an embeddedsusceptor, comprising: (a) an electromagnetic pulse generator togenerate pulses to vibrate the susceptor; (b) a receiver for receivingan acoustic signal generated by the vibrating susceptor; and (c) ananalyzer for discriminating the strength of the weld or bond from theacoustic signal.
 7. An acoustic signal stored in analog or digital formon suitable recording media, the signal representing the vibration of anembedded susceptor in a thermoplastic weld or adhesive bond arising frompulsing the susceptor inductively with an electromagnetic pulse testsignal.
 8. A method for nondestructively determining that a structuralbond being a thermoplastic weld or a structural adhesive bond hasadequate strength, the weld or bond including an embedded susceptor, themethod comprising the step of: (a) vibrating the susceptor inductivelywith an electromagnetic test pulse to produce acoustic vibrations; and(b) identifying that the acoustic vibrations lack low frequency acousticvibrations indicative of low strength structural bonds in the 1-3 kHzrange.
 9. The method of claim 3 wherein the susceptor is in athermoplastic weld joining a wingskin and a spar.
 10. A method forforming a thermoplastic weld between at least two, fiber-reinforcedcomposite laminates, comprising the steps of: (a) assembling at leasttwo laminates to define a bond line along faying surfaces of thelaminates; (b) positioning a susceptor along the bond line; (c) heatingthe laminates along the bond line to weld the laminates together byheating the susceptor; (d) nondestructively evaluating the weld qualityby analyzing acoustic signals generated by electromagnetic pulsesabsorbed in the susceptor; and (e) rewelding in at least those regionsof the weld found to have inadequate strength.
 11. A welded product madeby the process of claim 10.